In the field of seats or similar, like stools, it is well known that office chairs are often designed so that the body is supported in an ideal sitting position, where anatomical stresses are prevented in order to guarantee the user's comfort. These chairs are made of a frame with a cushion and a seatback. The seatback may be articulated to the frame with a return spring device to improve the comfort of the seat.
Such office chairs have the shortcoming of not exerting the muscles of the legs and of the back, which means that their users risk the degeneration of the spinal muscles and vertebral damages, such as wearing of the discs due to poor posture.
For good posture, it is well known to use a “Pezzi®” ball, for example. The user can sit on Pezzi® balls and use the muscles of the legs and the back.
However, the Pezzi® balls are particularly cumbersome, i.e., they are especially unsuitable for use in an office.
Many gymnastics devices are known, such as bicycles, leg presses and rowing machines, etc., to work the muscles of the legs and the abdominal muscles. However, in addition to not being usable at the office, these devices have the shortcoming of requiring movements, which restricts their use, either because the user will lack mobility or because of space requirements.
Devices are known, like electrostimulation, which allow isometric contraction of the muscles of the legs and of the abdominal muscles. However, they are difficult to use, particularly at the office, notably because putting the electrodes in place requires undressing in particular.
Many dynamic seats and stools have been designed to overcome all these shortcomings. They notably include the German patent applications DE 2314717 and DE 19533558.
German patent application DE 2314717 describes a stool with two ball and socket joints with springs, wherein the user is in a state of unstable balance, such that he can return to a normal position by means of a lateral deviation with actions by his legs.
German patent application DE 19533558 describes an active dynamic stool with a leg, whose base surface is an upward concavity curve, which allows a tilting movement, with a column secured to the leg, and a seat mounted on a fulcrum at the upper end of said column. The curved base surface of the leg lies directly on the ground and has a spherical curve on at least a partial zone. The seat is mounted at its fulcrum so that it can tilt to any side. Said fulcrum is either located under the center of the curvature of the base surface, or can be adjusted within a range of height values, whose lower limit is located under the center of the curvature. The seat is mounted on a junction element made of an elastic item that works as a spring and lies on a strut secured to the upper end of the column.
All these devices have the common shortcoming of being cumbersome and not allowing the isometric contraction of the muscle groups of the lower limbs, in particular.